In order to form refractory brick or like material it is necessary to fire shaped masses of granular ceramic material containing alumina and silica. These materials are naturally porous and expansible and are normally found naturally as the minerals diatomite, lavalite, expanded clay, glauconite, pearlite, and the like. Such blocks or bricks are formed by first imparting to the mass of granules of such material the desired shape, then firing the shaped masses to a temperature above the softening point of the material in an oxidizing or reducing atmosphere, normally by passing hot gas over the shaped masses. Thereafter the fired masses are cooled.
A considerable problem with this method is that the fired bricks or blocks stick very tightly to the support on which they stand while being fired. German Pat. No. 1,914,372 has suggested that this can be overcome by coating the mold in which the brick is formed with a powder, for example of sinterable clay, that fuses at a temperature much higher than that of the alumina- or silica-containing material being made into the bricks. Thus this lining powder keeps the material being fired out of direct contact with the mold so that subsequent separation is relatively easy. This system has the considerable disadvantage that the lining powder is relatively expensive, and that it is necessary to hold the mass being shaped in the lining until it is completely cooled. It is not possible to demold the object until it is relatively cool.
It has also been suggested to form the mold of metal alloys capable of resisting the extremely high heat employed during firing of the alumina- and silica-containing material. Since such alloys have an altogether different coefficient of thermal expansion from that of the material being shaped, it is not extremely difficult to demold the objects later. Nonetheless some material will inherently remain stuck in the mold so that eventually it will have to be chipped out or the expensive mold will have to be discarded. Furthermore it is necessary to separate the molded body from the mold at temperatures several hundred .degree.C. below the firing temperature so that considerable energy is wasted and substantial thermal stresses are applied to the mold.
It is also known to line the firing mold with a nonoxide ceramic such as boron nitride, silicon nitride, and the like. Such materials can, however, only be used in a reducing firing atmosphere as they are destroyed above 1000.degree. C. by oxygen. In addition these ceramics are extremely expensive so that they cannot be used in any practical application. In experimental use they have shown that the use of silicon nitride on the surface of the mold forms a passivating silicon oxide layer which can be scraped off by the material being fired so that even in experimental use the material is quickly used up.